Chapter 4. Vegetation
109
web page or the USDA-NRCS Plant Database (Reed 1988, USDA-NRCS 2001,
USFWS 2003). While these classes do not isolate the effect of flooding per se, in
arid regions they do indicate the overall reliance of a species on supplemental
moisture, which is likely to be well correlated with surface flow characteristics.
If riparian plants are classified into functional groups based on traits that
reflect adaptation to flooding, the spatial distributions of such functional groups
on a given floodplain might be helpful for identifying recent high water levels.
For example, "hydromesic ruderals" (e.g., Xanthium strumarium, Polygonum
lapathifolium, Polypogon monspeliensus) are disturbance-adapted herbaceous
plants with relatively high moisture requirements that occur in riparian zones in
Arizona (Stromberg and Bagstad, in prep). Such plants, by definition, are annuals
or biennials with wetland indicator classifications of Obligate Wetland through
Facultative Upland, meaning that they require more moisture than is normally
provided by direct precipitation. The zone of dominance of hydromesic ruderals
on the floodplain might therefore be indicative of the zone of regular flooding,
while the point at which herbaceous dominance shifts to more xeric ruderals
(e.g., Acalypha neomexicana, Bouteloua aristidoides, Sisymbrium irio) might
indicate the limit of the regularly flooded zone (Fig. 25) (Stromberg and Bagstad,
in prep)*. For perennial herbs, dominance by hydromesic rhizomatous or
stoloniferous species (e.g., Paspalum distichum, Cynodon dacytlon) may be
indicative of the zone of regular fluvial disturbance, since clonally reproducing
plants may readily recolonize floodplain sites following flood disturbance
(Stromberg 1993c) and may be better adapted to persist in the shade of young,
dense cottonwood stands (Friedman et al. 1996). Of course, such relationships
would need to be fine-tuned in order for such patterns to be used reliably as
indicators of specific levels of surface flow, and additional research into the
actual water sources (e.g., precipitation, surface flow, groundwater) used by
specific herbaceous plant species would be needed.
*
Also personal communication, J. Stromberg.
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